Primary graft dysfunction (PGD) is severe acute lung injury occurring in the days after lung transplantation and is characterized by diffuse pulmonary edema and profound hypoxemia. PGD has a major impact on outcomes following lung transplantation, markedly increasing early morbidity, mortality, and cost. Thus, reduction in the incidence of PGD would dramatically improve outcomes following lung transplantation. During the prior R01 cycle, we established donor smoking history, measured by communication through a proxy, as a significant risk factor for subsequent PGD of the allograft, as well as useful predictor of PGD. Concordant with our findings, cigarette smoke exposure has recently been shown to predispose to acute lung injury in the trauma population. However, a strategy of excluding donors with a smoking history from the lung donor pool leads to an overall increased mortality for those awaiting lung transplantation; therefore, better quantitative measurement of donor smoking on PGD risk, and enhanced understand the mechanisms by which smoking contributes to PGD are necessary to improve transplant outcomes. We hypothesize the effect of donor smoke exposure on PGD risk can be more fully quantified by biochemical measures, and that the increased risk of PGD in smoking donors is linked to alterations of the donor microbiome and effects on resident lung immune cell populations. To address these hypotheses, we will expand our research infrastructure established in the prior R01 cycle to define the quantitative relationship between donor smoke exposure and PGD; determine the relationships between alterations in resident lung immune cell populations, donor smoking status, and PGD; and define the relationships between allograft microbiome populations, donor smoking status, and PGD. Our aims will establish the role of urinary biomarkers of smoke exposure in donor risk stratification to safely expand the donor pool. As well, we will define the role of novel mechanisms in PGD risk that are amenable to therapies aimed at reducing the elevated risk seen in donors with smoke exposure.